The present invention relates to a thermal type flow sensor for measuring a mass flow rate of a fluid by using a heating resistor.
A conventional typical thermal type flow sensor has two temperature-dependent heating resistors incorporated in a bridge circuit. One of these resistors is used as a heating resistor for measuring a flow rate, and designed to have a relatively small resistance value because a heating current is applied. The other resistor is used as a temperature compensating resistor for a fluid to be measured, and designed to have a larger resistance value than the heating resistor because it is used at room temperature. In such a bridge circuit, potentials at two midpoints are sent to an operational amplifier and the current which flows through the heating resistor is controlled so that even when the heat of the heating resistor is lost according to the fluid flow rate (namely, flow velocity), the temperature difference between the heating resistor and the temperature compensating resistor is maintained at a prescribed value (the difference between the potentials at the midpoints is zero).
Furthermore, as one example of the prior art, Patent Document 1 (Japanese patent publication No. S-59-136620) discloses a flow rate measuring device which does not use a temperature compensating resistor but uses two bridge circuits comprised of only a heating resistor and plural fixed resistances where the heating temperatures of the heating resistors in both the bridge circuits are set to be different from each other. In this device, difference in output related to heating currents which flow through respective heating resistors of the two bridge circuits is sent to a microcomputer (microprocessor) to calculate the flow rate.
According to this prior art, the flow rate can be measured under no influence of fluid temperature without using a temperature compensating resistor. The flow rate measuring device described in Patent Document 1 has been proposed based on the following concept. The heating resistor and the temperature compensating resistor have different thermal time constants, which exerts an unfavorable influence on the flow rate measuring accuracy; therefore, in order to eliminate such an influence, a device which uses no temperature compensating resistor is proposed.
Furthermore, Patent Document 1 proposes a technical matter of disposing a protective heater on a base (plate) for supporting the heating resistor to heat the base only. This protective heater is independent from the heating resistor for measuring a flow rate (measuring heating resistor), and it is controlled to become the same temperature T1 as the measuring heating resistor. The protective heater is used to prevent transfer of heat from the measuring heating resistor to the base and improve the response of the measuring heating resistor.
The support for supporting the flow rate-measuring heating resistor, in a case of using a hot wire as the heating resistor, is comprised of a tube made of insulating material (for example, alumina tube) for winding the hot wire.
The support is maintained at a certain temperature level by heat transfer from the heating resistor. More specifically, since the support's portion other than the heating resistor winding area is exposed to the fluid to be measured and the heat of the support is dissipated into the atmosphere of the fluid, the support is maintained at a certain temperature level lower than the heating resistor.
If the temperature level of this support is kept almost constant (in other words, the level of heat dissipation from the support to the atmosphere of the fluid to be measured is kept almost constant), the level of heat dissipation through the heating resistor support is almost constant and this heat dissipation does not unfavorably affect the flow rate measuring accuracy.
A flow rate measuring device using a heating resistor has been generally used to measure an intake air flow rate of a vehicle internal combustion engine. In recent years, it has been sometimes used in an exhaust gas atmosphere in order to measure the EGR flow rate in an EGR (exhaust gas recirculation) system of a vehicle internal combustion engine.
When the flow rate measuring heating resistor is used in an exhaust gas atmosphere, soot-based nonvolatile matter contained in the exhaust air would gradually deposit on the support for supporting the flow rate measuring heating resistor. As such soot volume increases over time, heat transmission between the support and the fluid to be measured (gas to be measured) would change from that before such deposition, the result would exert an unfavorable influence on output of the fluid flow sensor. More specifically, as the rate of heat dissipation from the support to the atmosphere of the fluid to be measured changes depending on the degree of soot deposition, the rate of heat dissipation from the flow rate measuring heating resistor to the support would also change, so that the output value of the flow rate measuring heating resistor in relation to the gas flow rate before soot deposition would be different from that after soot deposition, resulting in a measurement error.
In order to prevent this, it is effective to stop the flow of heat from the flow rate measuring heating resistor to the support thereof.
In the structure described in Patent Document 1, the protective heater (hereinafter sometimes called the “second heating resistor”) is wound around the support for the flow rate measuring heating resistor (hereinafter sometimes called the “first heating resistor”) to become the temperature of the second heating resistor equal to the temperature of the first heating resistor. Thereby, heat transfer from the first heating resistor to the support is prevented and the response in flow rate measurement is improved.
When the temperature of the second heating resistor is made equal to the temperature of the first heating resistor, heat transfer from the first heating resistor to the support is prevented to the some level but actually it is impossible to prevent such heat transfer completely. More specifically, since the support's portion between the first and second heating resistors is exposed to the atmosphere of the gas to be measured, there is heat dissipation from the support to the atmosphere gas. Hence, when the temperature of the second heating resistor is made equal to the temperature of the first heating resistor, the temperature of the support becomes lower than that of the first heating resistor for measuring a flow rate. As a consequence, a heat flow from the first heating resistor to the support occurs.
Even if such a heat flow occurs, when the fluid to be measured is an air passed through an air filter, there is no soot deposition on the support and the level of heat dissipation from the support to the air is kept virtually at a constant level. Consequently, it does not exert an unfavorable influence on the flow rate measurement accuracy and makes it possible to achieve a response improvement as desired.
However, even in the flow rate measuring device as described in Patent Document 1, when the flow rate measuring element (which is comprised of first and second heating resistors mounted on the support) is placed in an exhaust gas atmosphere, soot deposition on the support might change the level of heat dissipation from the support to the exhaust gas atmosphere, exerting an unfavorable influence on the flow rate measurement accuracy.